1. Field of the Invention
The present invention relates to an electrical space heating element and in particular to a space heating element having an electrically resistive coating applied to the surface of a ceramic body on the side facing away from the space to be heated.
2. Description of the Prior Art
Space heating elements are generally provided under window openings so that the air of the room which is to be heated by the heating element rises past the window openings, thereby producing a curtain of warm air in front of the window. Also, in the case of radiators located against the walls, there is a movement of air from the floor to the ceiling of the room being heated, so that the heat radiated by the radiator into the room plays only a small part.
Conventional room heating elements of the radiant type have radiation areas which are very limited and must operate at high temperatures to be effective.
The prior art also includes floor heating devices, by which heating coils contained in a liquid heat transfer medium, or electrical heating conductors, are provided in the floor pavment or beneath the uppermost floor covering. However, floor heating is relatively troublesome and expensive to install and requires relatively complicated controls.
Accordingly, there exists a need for space heating elements for the periphery of the room which can perform singularly, in groups, or which cover the entire periphery of the room or a substantial part thereof, to heat the room or a selected part of the room.
Examples of such space heating elements are ceramic bodies, such as floor tiles, which act as the floor, wall tiles for the walls, and ceiling tiles for the ceiling. These tiles can be used to heat various spaces, such as private rooms, stores, offices, sanitary rooms, gymnasiums, and indoor swimming pools. Such ceramic bodies may also be shaped bricks for lining swimming pools and the like.
A pamphlet issued by Canespa KG 3005 Hemmingen-Westerfeld Gutenbergstrasse 13, in 1975, discloses a wireless heating system called "Canespa-therm" in which a heating varnish layer is provided as an electrically resistive coating on the back of a ceramic tile. This heating varnish layer is covered by a polyurethane foam body. However, this system did not find wide acceptance, since there was frequent local overheating resulting in various adverse consequences which even included safety risks to persons and objects.
The prior art also includes an electrical space heating device according to DE-A No. 1440971 which includes a carrier with a large smooth surface and a thin heating conductor extending across the surface and adhering directly thereto. The carrier is an earthware or vitreous fused silica tile. This tile is provided on its side facing away from the visible side with a thin metallic electrically conductive coating extending across the surface which is formed by precipitating a chemical solution or by applying a very thin aluminum layer.
It is necessary in the case of a space heating element of the foregoing type to provide for spacing between the tile and the carrier surface in which it is mounted and this spacing, in turn, forms a path for the air of the room behind the space heating element with all the disadvantages described above which have already been observed in the case of the space heating devices. Furthermore, the metallic resistive layers used have a relatively low resistance value and require high electrical power to obtain a sufficient thermal yield.
German Pat. No. DE-A 1924202 and the nonpublishd application DE-A No. 3325204 disclose a sheetlike electrical heating device which has a sheetlike carrier that includes a heating element, in which the heating element is formed as a thin layer of electrically conductive material which is applied to one surface of the carrier. In the event the electrically conductive material consists wholly or partially of semi-conductor material, as described in DE-A No. 3325204, uniform results cannot be achieved therewith because the electrical resistance of such layers cannot be produced in a reproducible manner. The heating device of this type, therefore, exhibits heating capacities which differ from piece to piece. German Pat. No. DE-A 1924202 discloses an electrically conductive glaze on the visible side of the ceramic body for removing static electricity. This glaze is so highly resistive that it is not suitable for heating purposes.
The ceramic bodies used up to now have maximum dimensions of 30 cm.times.30 cm because they can only be produced by the dry press method if the tile thickness is to be as small as possible. A large number of such tiles are required if a wall surface is to be covered with such tiles in order to heat a room. Correspondingly, a large number of electrical connections have to be made between the individual tiles. Further, the joints between the individual tiles create unheated gaps in the surface considerably reducing the portion of the covered wall available as radiating surface.